This invention relates in general to electrophotography and, in particular, to an electrophotoconductive imaging member having an electrically conductive ground strip layer.
In electrophotography, an electrophotographic plate containing a photoconductive insulating layer on a conductive layer is imaged by first uniformly electrostatically charging its surface. The plate is then exposed to a pattern of activating electromagnetic radiation such as light. The radiation selectively dissipates the charge in the illuminated areas of the photoconductive insulating layer while leaving behind an electrostatic latent image in the non-illuminated areas. This electrostatic latent image may then be developed to form a visible image by depositing finely divided electroscopic marking particles on the surface of the photoconductive insulating layer. The resulting visible image may then be transferred from the electrophotographic plate to a support such as paper. This imaging process may be repeated many times with reusable photoconductive insulating layers.
An electrophotographic imaging member may be provided in a number of forms. For example, the imaging member may be a homogeneous layer of a single material such as vitreous selenium or it may be a composite layer containing a photoconductor and another material. One type of composite imaging member comprises a layer of finely divided particles of a photoconductive inorganic compound dispersed in an electrically insulating organic resin binder. U.S. Pat. No. 4,265,990 discloses a layered photoreceptor having separate photogenerating and charge transport layers. The photogenerating layer is capable of photogenerating holes and injecting the photogenerated holes into the charge transport layer.
Other composite imaging members have been developed having numerous layers which are highly flexible and exhibit predictable electrical characteristics within narrow operating limits to provide excellent images over many thousands of cycles. One type of multilayered photoreceptor that has been employed as a belt in electrophotographic imaging systems comprises a substrate, a conductive layer, a blocking layer, an adhesive layer, a charge generating layer, a charge transport layer and a conductive ground strip layer adjacent to one edge of the imaging layers. This photoreceptor may also comprise additional layers such as an anti-curl back coating and an optional overcoating layer.
Imaging members are generally exposed to repetitive electrophotographic cycling which subjects exposed layers of imaging devices to abrasion, chemical attack, heat and multiple exposures to light. This repetitive cycling leads to a gradual deterioration in the mechanical and electrical characteristics of the exposed layers. For example, repetitive cycling has adverse effects on exposed portions of the imaging member, such as the ground strip, charge transport layer, and anti-curl back coatings. Attempts have been made to overcome these problems. However, the solution of one problem often leads to additional problems.
For example, in order to image an electrophotographic imaging member, the conductive layer must be brought into electrical contact with a source of fixed potential elsewhere in the imaging device. This electrical contact must be effective over many thousands of imaging cycles in automatic imaging devices. Since the conductive layer is frequently a thin vapor deposited metal, long life cannot be achieved with an ordinary electrical contact that rubs directly against the thin conductive layer. One approach to minimize the wear of the thin conductive layers is to use a grounding brush such as that described in U.S. Pat. No. 4,402,593. However, such an arrangement is generally not suitable for extended runs in copiers, duplicators and printers
Another approach to improve electrical contact between the thin conductive layer of flexible electrophotographic imaging members and a grounding means is the use of a relatively thick electrically conductive grounding strip layer in contact with the conductive layer and adjacent to one edge of the photoconductive or dielectric imaging layer. Generally, the grounding strip layer comprises opaque conductive particles dispersed in a film forming binder. This approach to grounding the thin conductive layer increases the overall life of the imaging layer because of its increased durability. However, such a relatively thick ground strip layer is still subject to abrasion and contributes to the accumulation of undesirable debris. In high volume imaging devices, abrasion is particularly severe in electrophotographic imaging systems utilizing metallic grounding brushes or sliding metal contacts because they often cause the ground strip layer to wear through. Moreover, wear in the ground strip layer may allow light to pass through the ground strip layer in systems utilizing a timing light in combination with a timing aperture for controlling various imaging functions, resulting in false timing signals which causes premature imaging process shut down.
U.S. Pat. No. 4,664,995 discloses an electrostatographic imaging member utilizing a ground strip. The disclosed ground strip material comprises a film forming binder, conductive particles and microcrystalline silica particles dispersed in the film forming binder, and a reaction product of a bi-functional chemical coupling agent which interacts with both the film forming polymer binder and the crystalline silica particles. However, such particles may agglomerate, resulting in impurities being trapped and in uneven optical properties.
Yoshizawa et al U.S. Pat. No. 4,717,637 discloses a microcrystalline silicon barrier layer.
Yoshizawa et al U.S. Pat. No(s). 4,678,731 and 4,713,308 disclose microcrystalline silicon in the photoconductive and barrier layers of a photosensitive member.
Tanaka U.S. Pat. No. 4,675,262 discloses a charge transport layer containing powders having a different refractive index than that of the charge transport layer excluding the powder material. The powder materials include various metal oxides.
Oguchi et al U.S. Pat. No. 4,647,521 discloses the addition of amorphous hydrophobic silica powder to the top layer of a photosensitive member. The silica is of spherical shape and has a size distribution between 10 and 1000 Angstroms. Hydrophobic silica is a synthetic silica having surface silanol (SiOH) groups replaced by hydrophobic organic groups such as --CH.sub.3.
If a relatively great frictional force acts between the photosensitive member and a cleaning member, the surface of the photosensitive member may be damaged, and wear off or filming of the toner may result due to the high surface contact friction between the cleaning device and the charge transport layer of the photosensitive member. Wear in the photosensitive member surface caused by high frictional force during machine function reduces the thickness of the charge transport layer. This reduction in charge transport layer thickness increases the electrical field across the layer, and alters electrophotographic performance. Moreover, static electricity generated by friction results in nonuniform surface potential in the charging step, which in turn causes an irregular image formation or fogging. In order to reduce the frictional force, the pressure of the cleaning member, e.g., a cleaning blade, may be reduced. However, by reducing the frictional force, the cleaning blade may not be able to clean the photosensitive member sufficiently, resulting in toner build-up or surface filming.
Other attempts at reducing the frictional force acting between the cleaning blade and the photosensitive member include adding a lubricant such as wax to the toner. However, the fixability of the toner may degrade its electrical function, or further filming may occur, resulting in a degraded image.
A further proposal for reducing frictional force involves applying a lubricant on the surface of the photosensitive drum. Kohyama et al U.S. Pat. No. 4,519,698 discloses a waxy lubricant method to constantly lubricate a cleaning blade. However, the thickness of the lubricant film formed on the photosensitive drum cannot be maintained, and interference with the electrostatic characteristics of the photosensitive member occurs. Attempts have also been made to construct a cleaning blade with a material having a low coefficient of friction. However, these attempts are subject to the problem of degradation in other characteristics, especially mechanical strength, due to the presence of additives.
Another problem in multilayered belt imaging systems includes cracking in one or more critical imaging layers during belt cycling over small diameter rollers. Cracks developed in the charge transport layer during cycling are a frequent phenomenon and most problematic because they can manifest themselves as print-out defects which adversely affect copy quality. Charge transport layer cracking has a serious impact on the versatility of a photoreceptor and reduces its practical value.
When one or more photoconductive layers are applied to a flexible supporting substrate, it has been found that the resulting photoconductive member tends to curl. Coatings may be applied to the side of the supporting substrate opposite the photoconductive layer to counteract the tendency to curl. How ever, difficulties have been encountered with these anti-curl coatings. For example, photoreceptor curl can sometimes still be encountered in as few as 1,500 imaging cycles under the stressful conditions of high temperature and high humidity. Further, it has been found that during cycling of the photoconductive imaging ember in electrophotographic imaging systems, the relatively rapid wear of the anti-curl coating also results in the curling of the photoconductive imaging member. In some tests, the anti-curl coating was completely removed in 150,000 to 200,000 cycles. This wear problem is even more pronounced when photoconductive imaging members in the form of webs or belts are supported in part by stationary guide surfaces which cause the anti-curl layer to wear away very rapidly and produce debris which scatters and deposits on critical machine components such as lenses, corona charging devices and the like, thereby adversely affecting machine performance. Also, the anti-curl coatings occasionally separate from the substrate during extended cycling and render the photoconductive imaging member unacceptable for forming quality images. It has also been found that when long webs of a flexible photoconductor having an anti-curl coating on one side of a supporting substrate and a photoconductive layer on the opposite side of the substrate are rolled into large rolls, dimples and creases form on the photoconductive layer which result in print defects in the final developed images. Further, when the webs are formed into belts, segments of the outer surface of the anti-curl belt in contact with each other during shipment or storage at elevated temperatures also cause creases and dimples to form which are seen as undesirable aberrations in the final printed images. Expensive and elaborate packaging is necessary to prevent the anti-curl coating from contacting itself. Further, difficulties have been encountered in continuous coating machines during the winter manufacturing of the coated photoconductive imaging members because of occasional seizing which prevents transport of the coated web through the machine for downstream processing.
Anti-curl layers will also occasionally delaminate due to poor adhesion to the supporting substrate. Moreover, in electrostatographic imaging systems where transparency of the substrate and anti-curl layer are necessary for rear exposure to activating electromagnetic radiation, any exposure to activating electromagnetic radiation or any reduction of transparency due to opacity of the supporting substrate or anti-curl layer will cause a reduction in performance of the photoconductive imaging member. Although the reduction in transparency may in some cases be compensated by increasing the intensity of the electromagnetic radiation, such increase is generally undesirable due to the amount of heat generated as well as the greater costs necessary to achieve higher intensity. An anti-curl layer which exhibits the above deficiencies is highly undesirable.
Thus, it is desirable to increase the durability and extend the life of exposed surfaces in an imaging device as well as to reduce frictional contact between members of the imaging device while maintaining electrical and mechanical integrity.